Relating to the comment you put at the top of every post, I do think of Kepler and Descartes doing the best work during the awfulness of the 30 years war. Good science matters in bad times. Maybe more.
I believe that residual magnetism in rocks was one of the clues towards continental drift. Does this imply that the natural lodestone forms magnetic field directed towards the current magnetic North? Or maybe the residual magnetism is in some more subtle form. Especially interested as I've just been inside an MRI machine... The Chinese called their first lodestones " navigational spoons"
Actually, I back off on my previous answer. It seems that (as seems logical) the magnetic field induced in rocks by lightning strikes (by lightning's electrical field, as Ron notes below) does indeed follow the geometry of the lightning, not the present earth's field. But what happens is the lightning strike induces a field so strong that it creates a permanent magnet - one that is then susceptible to the whole earth's field. So a shred of lodestone, like a piece of a permanent synthetic magnet, will orient in the present weak earth's field simply because it is a permanent magnet. It's irrelevant what the geometry was of the outcrop of magnetite that the lightning hit - what matters is that a strong, local permanent magnet was created. It would then take a tiny, rotatable piece of that magnet to serve as a compass. The lodestone in my photo is far too big for the weak earth's field to twist or otherwise orient it.
Just done more careful thinking through with a bit of help from Google Gemini.The magnetism produced by lightning current is direct result of ampere's law- lines of magnetic force circling around an electric current.
A few mm from the lightning current the magnetic field strength is eight teslas or two MRI machines. Oriented perpendicular to the lightning current, irrespective of the Earth's vastly weaker field. Paleo magnetism, also in magnetite, very very weekly magnetized by the earth's field as the crystallization passes through 580°. According to Gemini that temperature! Very precise, but I forgot to note whether that's degrees c or degrees f...
That's degrees C, and it is called the Curie temperature, the temperature where the material (magnetite) loses (or gains) magnetism. Other magnetic materials, such as nickel, have different Curie temperatures. What it means for rocks is that if a rock that gained a magnetic orientation as it solidified (cooled) in an older earth's field (paleomagetism, remanent magnetism), it can be re-set, changed, if something heats the rock up again past 580° in a different (more recent) earth's field which is then imposed on the rock when it cools a second time.
Re magnetism and plate tectonics, the discovery (in rocks like volcanic flows on the surface) that the induced field in the rocks showed reversals helped drive the understanding of the alternating high (normal) and low (reversed) "stripes" in the magnetic field symmetrically around mid-ocean ridges. That led directly to the idea of sea-floor spreading, and it was hardly a leap from that to true plate tectonics.
That's a great question. While one might think that if lightning is the cause, the created magnetic field might be related to the geometry of the lightning strike, but it seems that lodestones do orient with the present whole earth's field (otherwise they would not be useful for navigation!). So, while I have not yet found any study regarding this, I bet what happens is the electric field from the lightning does something to the magnetic domains within the magnetite grains so that the (weak) earth's field is able to induce an orientation in those domains related to the earth's field orientation. Edit for future readers: see my later reply for a much more correct answer as well as the other discussions.
Relating to the comment you put at the top of every post, I do think of Kepler and Descartes doing the best work during the awfulness of the 30 years war. Good science matters in bad times. Maybe more.
I believe that residual magnetism in rocks was one of the clues towards continental drift. Does this imply that the natural lodestone forms magnetic field directed towards the current magnetic North? Or maybe the residual magnetism is in some more subtle form. Especially interested as I've just been inside an MRI machine... The Chinese called their first lodestones " navigational spoons"
Actually, I back off on my previous answer. It seems that (as seems logical) the magnetic field induced in rocks by lightning strikes (by lightning's electrical field, as Ron notes below) does indeed follow the geometry of the lightning, not the present earth's field. But what happens is the lightning strike induces a field so strong that it creates a permanent magnet - one that is then susceptible to the whole earth's field. So a shred of lodestone, like a piece of a permanent synthetic magnet, will orient in the present weak earth's field simply because it is a permanent magnet. It's irrelevant what the geometry was of the outcrop of magnetite that the lightning hit - what matters is that a strong, local permanent magnet was created. It would then take a tiny, rotatable piece of that magnet to serve as a compass. The lodestone in my photo is far too big for the weak earth's field to twist or otherwise orient it.
Just done more careful thinking through with a bit of help from Google Gemini.The magnetism produced by lightning current is direct result of ampere's law- lines of magnetic force circling around an electric current.
A few mm from the lightning current the magnetic field strength is eight teslas or two MRI machines. Oriented perpendicular to the lightning current, irrespective of the Earth's vastly weaker field. Paleo magnetism, also in magnetite, very very weekly magnetized by the earth's field as the crystallization passes through 580°. According to Gemini that temperature! Very precise, but I forgot to note whether that's degrees c or degrees f...
That's degrees C, and it is called the Curie temperature, the temperature where the material (magnetite) loses (or gains) magnetism. Other magnetic materials, such as nickel, have different Curie temperatures. What it means for rocks is that if a rock that gained a magnetic orientation as it solidified (cooled) in an older earth's field (paleomagetism, remanent magnetism), it can be re-set, changed, if something heats the rock up again past 580° in a different (more recent) earth's field which is then imposed on the rock when it cools a second time.
... Because it wouldn't do to make it all too obvious... And I note some nice sedimentary paleo magnetism there on your North Slope.
Re magnetism and plate tectonics, the discovery (in rocks like volcanic flows on the surface) that the induced field in the rocks showed reversals helped drive the understanding of the alternating high (normal) and low (reversed) "stripes" in the magnetic field symmetrically around mid-ocean ridges. That led directly to the idea of sea-floor spreading, and it was hardly a leap from that to true plate tectonics.
That's a great question. While one might think that if lightning is the cause, the created magnetic field might be related to the geometry of the lightning strike, but it seems that lodestones do orient with the present whole earth's field (otherwise they would not be useful for navigation!). So, while I have not yet found any study regarding this, I bet what happens is the electric field from the lightning does something to the magnetic domains within the magnetite grains so that the (weak) earth's field is able to induce an orientation in those domains related to the earth's field orientation. Edit for future readers: see my later reply for a much more correct answer as well as the other discussions.